Cathode-ray tube for reproducing coloured television images



P. SCHAGEN 2,730,653

CATHODE--RAY TUBE FOR REPRODUCING COLOURED TELEVISION IMAGES Jan. 1o, `1956 Filed March 18, 1955 E United States Patent` f CATHODE-SRAY TUBE FOR REPRODUCING` i COLOURED TELEVISION IMAGES Pieter Schagen,.Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, l i Conn., as trustee Application March 18, 1953, .Serial No. 343,139 i r,Claims priority, application Netherlands April 17, 1952 4 ClaimS. (Cl. 315-15) The invention relates Vto cathode-ray tubes for reproducing coloured television images and also to devices comprising such tubes. Widely different suggestions have ybeeny made for the purpose ofv reproducing coloured television images. Prior systems generally used coloured fil- `ters and optico-mechanical systems. Modern systems often use cathode-ray tubes comprising a luminescent screen which by special measures is adapted to reproduce coloured television images. One of the latest tubes comprises a luminescent screen on which three luminescent materials are point-distributed, each of the materials -luminescing4 in a primary colour. The said points are excited Aby the electron beam and the aggregate is operated in a Vmanner such that the general impression ofthe observer is a coloured image. To ensure suicient denition of the image a great many points are required. Obviously the manufacture of such a'screen was looked upon as very -diiiicu'lt so that endeavours were `made to devise other forms of screen construction in which no points of luminescent `materials are required. According to one ofthe solutionsv of this problem which were thought to have been found the luminescent materials are not provided in points, butin the shape of juxtapositioned strips of luminescent material, each strip luminescing in a primary colour.

In both the above described systems it is very diicult to -direct the electron beam to the correct point or the cor- 'rectistrip respectively. Many means have been suggested `for this' purpose, among other. things' `the provision in front ofthe luminescent screen of deiiectng grids or of an electrode comprising a large number of holes. Though these expedients provide satisfactory scanning, they have l a limitation in that they require highlyv satisfactory correlation between the luminescent screen and the electrode arranged in front thereof. When manufacturing the .tube considerations Yof minimized tolerances in the distances between the screen and the electrode arranged before it, `between the points or lines respectively on the screen and between the apertures of the electrode arranged in front thereof must be met. To obviate this difliculty other means have been looked for and it has been suggested to apply 'apdenite potential to the luminescent materials of one colour, for example by applying them to a conductive underlayer. By then varying the potentials of the electrodes with the luminescent material it ispossible to direct thebeam atv will to a definite electrode and thus to produce light of a definite colour. Obviously such a system becornes substantially impossible if a screen is point coated with dilerent luminescent materials. If the luminescent materialsV are arranged in strips no particular diiculties occur, so long as no more than two luminescent materials which luminesce in different colours are used. The Astrips enter into each other like combs and current may be supplied to the back of the combs. With more than two lumi'- nescent materials luminescing in. different colours it is very diiiicult to establish a connection. with. all the strips. In. a system comprising three primary colours the connections i 2,730,653 Patented Jan. 10, 1956 may ,be established as 'described hereinbefore for v'two groups of strips, butparticular measures. lare required for the group of strips for the third colour. "Thusyfor example, connection may be made across thegside,Y all the strips being connected .in series." However, this produces a heavy resistance between points of connection andthe strips' vw'dely remote therefrom. As an'alternative, the connectionsof the strips may cross each other in an insulated manner. However, this produces excessiveV capacity between. the electrodes of different colours. l

A further alternative method enables in a simple manner, without the use of complicated electrode structures, to cause a screen comprising a plurality .of luminescent materials to emit a denite colour by varying the potential of a single electrode, the luminescent screen being constituted by a plurality of superposed'l'ayers luminescing in. different colours. Atpthat 4side of such a screen which is more remote yfrom the cathode provision is made of an electrode to which varying potentials are applied. The thicknesses and the electron permeabilities of lthe luminescent layers are `such, that on `the potentials of the electrodes arrangedl behind the screen being Varied the electrons penetrate one, two or'more layers. it is obvious that, when the potential is suiiiciently high for the electrons to penetrate the first layer viewed from the cathode and to excite one or more of the subsequent layers, the first layer will also emit light. Consequently, a mixed colour is thus invariably produced. lf only two ylayers are used, the thickness of these layers antil more .particularly the thickness of the rst, layer may still. be such that a tolerable result is achieved. However, with the use of more than 4two' layers, for .example three, as is almost common practice, there can be hardly any ques- Vtion of true colour reproduction.

The two systems describedhereinbefore have an yadditional limitation in that the necessary variation of the potential of the electrode to which the luminescent material is applied causes the deection of the Yelectron Vbeam to varyas well. To provide compensation for this the deecting potential is required to be varied in the rhythm of the vpotential variations of the above mentioned electrodes.

The object of the invention is to obviate the above mentioned disadvantages of known systems by a different structure of the luminescent screen and theV associated electrodes.

A cathode-ray tube according. to the invention for reproducing coloured television images has an electrode system including a luminescent screen, and is characterized in. that at the cathode side of this screen provision is made of agrid-shaped electrode which at the observed side is coated with a. material whichby the bombardment of electrons luminesces in colour other than the lrst-men- .tionedluminesceut screen and in that at the observer side of the grid-shaped electrode provision is made of a field electrode.

The operation of a cathode-ray tube according to the invention is based on that the field electrode may have supplied to` it an alternating voltage which either sends back the* electrons to the grid-shaped electrode, where they excite the luminescent material provided on it, or sends the electrons to the luminescent material provided on the screen. v

There are still several possibilities of constituting the luminescent screen and arranging the field electrode, since this electrode may be arranged before, in or behind the luminescent screen. Being arranged between the luminescent grid-shaped electrode and the observer it is required either to be grid-shaped or to consist of ametal layer which is permeable to light and consequently also to electrons.

, The luminescent screen may be constituted either by the comb described hereinbefore or by the above described construction comprising two superposed 'luminescent layers. If two primary colours are considered sufficient, the constructionV is very simple indeed, but even in the -c'as'e of three'primary colours thestruetural4 difiiculties l'maybe readily overcome and highly satisfactory colour reproduction is obtainable. A

In a Yparticular case the luminescent material maybe rendered sutciently conductive, for example by mixing it with a metal, forV enabling the potential to be applied to'thescreen itself. In many cases the lield electrode may be combined with lthe luminescent screen so'as to form a single unit.

The invention will'now be described with reference to the accompanying drawing, in'which:

Fig. 1 is'an elevation and a partial cross-section of a cathode-ray tube using two primary colours;

coated thereon are designated by the reference numerals 13, 14, 15 and 16 respectively.

Fig. 2 is a detail-view of the tube according to Fig. 1; Y'

Fig. 3 shows an alternative embodiment of the screen construction according to Fig. l;

Fig. 4 is a detail-view of a modified construction of a tube according to Fig. l;

Fig. 5 is a detail-view of a tube using three primary colours;

Fig. 6 is a detail-view of a cathode-ray tube in which a comb-shaped construction of the luminescent screen is used.

Referring now to Fig. l, the reference numeral 1 desv ignates the envelope of a cathode-ray tube, which has a glass end face or window 2. On the inner surface of the window 2 provision is made of a transparent conductive layer 3 which is made for example of conductive tin oxide. At the cathode side this layer is coated with a luminescent layer 4. A grid-shaped electrode 5 is arranged in front of this layer 4 andslightly spaced away from it. As may be seen more clearly from Fig. 2, this grid-shaped electrode is coated at its side more remote from the cathode with a luminescent material 6, which by the bombardment of electrons emits light of a colour diierent from that emitted by the layer 4.

The operation of the tube may be readily explained with reference to Fig. 2. The beam-forming system comprises a cathode 30, a control grid 31 and an anode 32, all connected to a suitable D.C. potential source 33. The electrode 5 has a definite potential applied to it, for example by connecting it to the anode ofthe electrode system which ensures the production of the beam. The electrode 3 is provided with a terminal to permit a definite potential from a square wave A.C. source 34 to be applied to it. When this potential is sufliciently negative with respect to the electrode 5, the electrons passing through the grid 5 will turn back before being capable of reaching the layer 4, so that they strike the luminescent material 6 and thus produce light. When the potential of the electrode 3 is higher than the potential of the grid 5, the luminescent layer 4 will be struck and emit light. Varying the potential of the electrode 3 consequently permits of generating either light having the colour which the material 6 is able to emit or light having the colour which the material of the layer 4 is able to emit.

In the construction according to Fig. 3 the luminescent material 8 is coated on a transparent underlayer 7. At the cathodes side this material 8 is coatedwith a conductive layer 9 which fulfills the same function as the layer 3 of Figs. 1 and 2. In this case the grid-shaped electrode is designated by the reference numeral 10 and the luminescent material provided on it by the reference numeral 11. Since the layer 9 is required to be permeable to light it is naturally also permeable to electrons.

Fig. 4 shows an embodiment which is similar to the one shown in Fig.' 3 except for the conductive layer 9 shown in Fig. 3 which is replaced by a grid-shaped electrode 12 acting in quite the same manner. The underlayer of the screen, the luminescent layer coated thereon, the grid-shaped electrode and the luminescent material Fig. 5 is a detail-view of a discharge tube for the emission of coloured images with the use of three primary colours. The transparent support 17 of the luminescent screen is coated with a light-transmitting conductive layer 18. This field electrode is coated at the Vcathode side with a layer of luminescent material 19 which by bombardment of electrons emits a definite colour. This layer is provided with a second luminescent layer 20, which on being struck by electrons luminesces in colour other than the material of the layer 19. The reference numeral 21 designates the grid which is arranged in front of the screen and is coated on the side more remote from the cathode with a` luminescent material 22 which emits light of a color other than emitted by the layers 19 and 20 respectively. lf the layer 18 has applied to it a potential which is suiiciently below the potential of the electrode 21 for the electrons passing through the grid to return, the electrons are enabled to excite the material 22. If

the potential of the electrode 18 exceeds the potential of the grid 21, it lis possible for the electron beam to reach the layer 20 and to excite it, the electrons being braked and absorbed. If the potential of the electrode 18 is very high, it will be possible for the electrons to penetrate the layer 20 and to excite the layer 19. At a less high potential substantially all the electrons will be absorbed in the layer 20 and consequently the layer 19 will not emit light. As mentioned hereinbefore, such a screen consisting of two superposed luminescent layers enables a highly satisfactory, only slightly contaminated colour image to be obtained. Generally for this purpose the layer 20 will be required to be much thinner than the layer 19 and the luminescent materials will be such that the layer 19 emits green light, the layer 20 red light and the layer 22 blue light.

Fig. 6 is a detail-vew on a large scale of a cathode-ray tube which acts with a comb-construction of the luminescent screen. The reference numeral 23 designates the y grid-shaped electrode, which at its side more remote from the cathode is coated with a luminescent material. The reference numeral 24 designates a transparent Acombshaped electrode which is coated with a luminescent mav terial which by the bombardment of electrons emits light of a colour other than that emitted by the material on the grid 23. The reference numeral 25 designates a combshaped electrode which is also transparent and which at the cathode side is coated with a third luminescent material. The operation of this cathode-ray tube may be described as follows. The grid 23 is at a definite positive potential. When both the electrodes 24 and 25 are nega- Y23 through the intermediate spacing. The grid 23 may be suficiently fine meshed to permit the luminescent screen 23 to be observed through every slit between the electrodes 24 and 25. If desired this permits of the electrodes 24 and 25 being impermeable to light but permeable to electrons and being coated with the luminescent materials at the side more remote from the cathode. In this event the light of the electrode 23 is visible through the slits only. Any colour contamination is thus prevented.

Obviously an alternative system which permits of building Aup two diterentlycoloured images may be provided atthe observer side of the grid-shaped electrode, Therefore i'the invention is not restricted to the embodiments shown but only by the requirement that a gridshaped electrode should be available, which at the side more remote from the cathode is coated with a luminescent substance and by the requirement that a tield electrede should be available at the observer side of this gridshaped electrode.

What I claim is:

1. A cathode-ray tube for reproducing color television images comprising an electron gun including a cathode, a transparent support facing said cathode, a rst layer of luminescent material lnrninescing in a first color disposed on the side of said support facing said cathode, a second layer of luminescent material luminescing in a second color superposed on said first layer, a grid-like electrode disposed between said luminescent layers and said cathode and in close proximity to the former, a third layer of luminescent material luminescing in a third color disposed on the side of said grid electrode facing said first and second luminescent layers, and a field electrode disposed between said third layer and said transparent support.

2. A tube as claimed in claim l wherein the second layer is electron permeable and is thinner than said first layer, and the tield electrode is disposed between said first layer and said support.

3. A tube as claimed in claim 2 wherein the first, second and third luminescent layers exhibit green, red and blue luminescence, respectively.

4. A system comprising a cathode-ray tube for reproducing color images comprising an electron gun including a cathode and anode, a transparent support facing said cathode, a first layer of luminescent material luminescing in a rst color disposed on the side of said support facing said cathode, a second layer of luminescent material luminescing in a second color superposed on said tirst layer, a grid-like electrode disposed between said luminescent layers and said cathode and in close proximity to the former, a third layer of luminescent material luminescing in a third color disposed on the side of said grid electrode facing said first and second luminescent layers, and a field electrode disposed between said rst rayer and said transparent support, a source of direct current potential, means for applying a given positive potential to said anode and the same positive potentiel to said grid electrode, a source of alternating current potential, means for applying an alternating current potential to said vlield electrode including a t'irst potential below said given potential, a second potential above said given potential and a third potential above said second potential, whereby application of the first, second and third potentials to the field electrode enables the third, second and first luminescing layers, respectively, to be struck by an electron beam.

References Cited in the le of this patent UNITED STATES PATENTS 2,461,515 Bronwell Feb. 15, 1949 2,580,073 Burton Dec. 25, 1951 2,590,764 Forgue Mar. 25, 1952 

